To estimate a maximum purchase price for the coin in question, we can analyze the provided data and consider several factors:

1. Current Condition: The coin has a condition rating of 80 (MBC+), indicating that it is in quite good shape. This generally translates to a higher price compared to coins with lower conditions.

2. Auction History:
- The coin has been auctioned 20 times and has sold on all occasions, suggesting consistent demand.
- Previous hammer prices for coins with similar conditions (80-90) range from 20 to 50 euros, with some exceptional cases reaching up to 125 euros in higher condition (90).

3. Starting Price and Average Price:
- The current starting price is 40 euros, which is above the average starting price of 28.07 euros and the average hammer price of 34.21 euros. This may indicate that the auctioneer expects greater interest in this particular coin.

4. Price Trends:
- Looking at previous hammer prices, the highest recorded price for a coin with a condition of 90 was 125 euros. However, since the condition of the coin in question is 80, it is reasonable to adjust this price downward.

5. Final Estimate:
- Considering all these factors, an estimated maximum purchase price could be in the range of 50 to 70 euros. This range takes into account the good condition of the coin and the price trends in previous auctions.

Therefore, an estimated maximum purchase price for this coin could be approximately 70 euros.